When communication signals are utilized in a system, a filtering function or an offset cancellation function is conventionally used to perform DC offset correction on the received signals. If the DC offset is not removed, certain circuitry coupled to receive the AC components of the signals may have an input voltage limit exceeded, have an overvoltage error, or otherwise not operate properly. The receiving circuit may not be able to use the full dynamic range of the receiver if an unwanted DC offset voltage is present at the input to the receiver.
In one known approach, an RC or high pass filter is used. The high pass filter circuitry may be described as performing an “AC coupling” function in that the output from the high pass filter circuitry should include any time changing signal components that are present in the input signals, the “AC” signals, while removing the DC voltage components. In a conventional approach, a passive filter is often used with a series coupled resistor capacitor circuit to remove the DC portion of the input signals. However, the conventional passive filter approach relies on a high pass filter that also attenuates any low frequency components that are present in the input signal. In an application where a wide frequency band contains needed information in the input signals, the high pass filter removes or significantly attenuates low frequency portions of the signals, and this attenuation results in a loss of information. Further, the low frequency attenuation sometimes also results in unwanted distortion in the output signals.
In another alternative known prior approach, dynamic offset cancellation is used to perform DC offset correction. In this approach, an offset voltage is sampled and stored, for example in a capacitor, during an offset sampling phase. During this offset sampling phase, switches or other control circuitry are used to remove the time varying input signal from the input to the DC correction circuit. In this manner, the output of the DC correction circuit is only the DC offset that is present during the offset sampling phase. The DC offset from the output can be stored by sampling the DC offset onto a capacitor or other storage element, and then during a signal processing phase, the stored offset can be subtracted from the input or from the output signal.
Disadvantages of the prior dynamic offset cancellation approach include that the input signal must be stopped from coming to the inputs or switches used to isolate the time varying input from the circuit during the offset sampling phase. This requirement places additional constraints on the system design. Further the offset voltage that is sampled is restricted to only the DC offset, and cannot include other low frequency components in the input signal, for example. The input signals are not received in the sampling phase so no information about the low frequency components of the input signal are available during the sampling phase.
In one example application, Optical Time Domain Reflectometry, (“OTDR”), the time domain response of a test signal including low frequency signals that include important information about a communications cable. In ODTR, one or more test pulses are transmitted to test the response of a communications cable. In this particular application, the cable is an optical cable. Reflections are then studied to characterize the line and identify any particular problem areas. FIG. 1 depicts an example response curve for an OTDR test. The figure also indicates the physical characteristics of the cable being tested that correspond to certain waveforms in the time domain response.
As shown in FIG. 1, a test apparatus 1 sends a test pulse and plots the response to the test pulse transmitted along an optical fiber. The plot shows attenuation of the test pulse versus distance along the cable. Various events visible in the resulting waveform correspond to different joints, connections, bends, splices or other physical characteristics of the optical fiber. The high frequency components in the signal waveform correspond to faults, bends and/or joints in the optical cable. The low frequency components correspond to signal losses such as “slow droop” and these low frequency signals provide additional information about the nature of the cable, including the cable signal loss rate for certain distances, for example.
In this application, it is important to cancel any DC offset at a receiver, so that the full dynamic range of the receiver circuit can be used. However, if the DC offset correction circuit in the receiver also filters out any low frequency components in the time domain response waveform, the information in the input signal that has components at low frequency, such as the signal droop information, will be lost.
Further, the response of the impulse/step function at the faults such as at joints, bends, or other faults in the optical fiber cable should not have an extended time domain response, as the extended response will spoil detection of nearby faults and joints, or nearby droop. The extended time domain response to impulse signals in the input will mask the waveform changes that are in the input signal due to these events, and since these events are then not visible in the output waveform, this extended time domain response will prevent these events from being observed.
FIG. 2 depicts, for the purpose of explanation, a circuit diagram of a typical prior art high pass filter DC offset correction circuit 10 used for differential signals. In FIG. 2, a positive signal and a complementary signal form a differential signal pair that are received at corresponding input terminals labeled INP and INM. The differential input signals are passed through a passive RC filter that has a symmetrical branch for each of the positive and complementary input signals that are coupled to terminals INP and INM. In FIG. 2, capacitor C1 is shown coupled between the input signal INP and a positive output terminal INPP for outputting the resulting output signal. Resistor R1 is coupled between an input node INCM for receiving a common mode input voltage, and a node N1 between the capacitor C1 and the output INPP.
Similarly, and in a symmetrical arrangement, the complementary input signal at terminal INM is coupled to capacitor C2, which is further coupled to an output terminal INPM for outputting the complementary differential output signal. A second resistor R2 is coupled between another input for receiving the common mode input voltage INCM and a node N2 that is between the capacitor C2 and the output terminal INPM.
The circuit 10 performs a high pass filter function. Circuit 10 will block DC offset in the input signal from the output signal. However, it is very difficult, even impossible, to support low frequency components in the input signals using this kind of prior art filter. The RC filters of circuit 10 not only attenuate the low frequency components of the input signal, but also cause distortion in the output. For example, step response transients take some time to die down in the output waveform. The delay in signal decay will corrupt later transient signals. For this reason, such RC filters are generally considered to be inappropriate for use in applications where low frequency signal support is needed along with DC offset correction.
FIG. 3 illustrates in a voltage waveform graph the response of the prior art pass filter shown in FIG. 2 to an example input signal labeled “INPUT”. In FIG. 3, the input signal illustrates a “burst” function switching between 0.0 and 1.0 Volts that occurs between time 0.0 and time 20 microseconds. The input signal INPUT then suddenly stops switching at time 20 microseconds. After that time the input signal INPUT remains at 0 Volts. A common mode input labeled INCM of 0.6 Volts is also shown in this example, and the output waveform labeled OUTPUT in FIG. 3 is approximately centered on this common mode input voltage.
FIG. 3 illustrates some of the problems of the conventional circuit approach (as shown in FIG. 2) for receiving a burst pattern in the input signal. The output voltage OUTPUT tracks the input voltage INPUT until the input waveform stops switching. Then, starting at time 20 microseconds, the output voltage waveform OUTPUT slowly rises towards the input common mode voltage INCM. The constant low voltage shown in the input signal starting at 20 microseconds is lost in the output waveform by the transient response to the stopped input signal, which distorts the output signal OUTPUT and covers up the true voltage (0 Volts after time 20 microseconds) at the input.
FIG. 4 illustrates in another voltage waveform graph a response of the conventional circuit shown in FIG. 2 to a slowly changing input waveform. As seen in FIG. 4, the conventional circuit 10 of FIG. 2 also fails to provide the correct low frequency response needed in this example. In FIG. 4, the input signal labeled INPUT is shown with a step down from about 1.0 Volts at time 0 microseconds to about 0.8 Volts at time 10 microseconds. The input voltage waveform INPUT then slowly falls from 0.8 Volts to 0.7 Volts at time 40 microseconds.
In FIG. 4, the output voltage waveform, again labeled OUTPUT, illustrates the attenuation of low frequency input signals in the conventional filter circuit 10 of FIG. 2. At time 10 microseconds, the output waveform OTUPUT tracks the input waveform, but then the output waveform again slowly rises until the end of the plot. The low frequency input signal that occurs in the input signal INPUT from time 11 microseconds to time 40 microseconds is not reproduced in the output signal OUTPUT. This low frequency information in the input signal is therefore not available for circuits that are coupled to the output of the pass filter 10, because the distortion in the output response masks the low frequency components. The information contained in the low frequency components of the input signal is simply lost. Improvements in DC offset correction circuits and methods for DC offset correction are therefore needed to address the deficiencies and the disadvantages of the known prior approaches.